1. Technical Field
The present invention relates to a servo controller which corrects position error when a moving member reverses.
2. Description of the Related Art
A servo controller is used for making a moving member accurately operate based on a position command. Such a machine is a machine tool which makes a moving member move along a feed shaft. In this regard, when the feed shaft is a threaded shaft of a ball screw, when the direction of movement of the moving member reverses, backlash or friction causes the reversal operation to be delayed. As a result, position error occurs.
In general, to correct delay at the time of reversal, the practice has been to add a position correction amount which corresponds to the backlash to the position command and to add a speed correction amount which compensates for the delay due to friction to the speed command.
However, when the feed shaft is a threaded shaft of a ball screw, the threaded shaft extends and contracts in the axial direction. Due to this, delay is caused at the time of reversal. To eliminate this, Japanese Patent No. 3621278 discloses a configuration provided with a mechanical end correction constant multiplying part which multiplies the torque which the electric motor generates by a constant factor and an adder which adds the total torque command value which is multiplied by the constant factor to the position command. In this case, it is possible to change the position command while considering the amount of extension or contraction of the threaded shaft due to the torque and as a result accurately correct the delay at the time of reversal.
However, the amount of extension or contraction of a threaded shaft changes not only due to the torque of the electric motor, but also the position of the moving member at the threaded shaft. In Japanese Patent No. 3621278, the position of the moving member is not considered, so there are limits to control of the position of the moving member with a high precision.
Further, to calculate the position correction amount with a high precision, it is necessary to prepare an optimal approximation equation based on the relationship between the amount of deformation of the threaded shaft, which includes the amount of extension or contraction, the torque of the electric motor, and the position of the moving member. However, if utilizing all of this data, calculation of the approximation equation becomes complicated and tremendous effort is required for the servo controller.
The present invention was made in consideration of this situation and has as its object the provision of a servo controller which can correct the position error by a high precision and in a simple manner when the moving member reverses.